The advantages of plastic containers in packaging over glass and metal containers is well known in the packaging art. However, utilization of plastic containers, in particular thermoplastic containers, in packaging presents some problems when the product to be packaged requires an elevated temperature during processing.
For example, some products are hot when filled into a container ("hot-filled"). Most comestible products are also pasteurized and/or sterilized once in the container (retort sterilization). Some products are heated by the consumer in the plastic package before consumption such as in boiling water or a microwave. These thermal events can cause problems with the integrity of the packaging.
Typically hot-filled applications involve heating products, usually comestibles, from above 140.degree. F. to about 190.degree. F., placing them in a container, and then sealing the container. Upon cooling of the product, the product shrinks in volume and thus a negative internal pressure results. Resulting pressure differentials create a net pressure force on the outside of the container wall, which can cause the container to buckle or collapse. Even more shrinkage of the product occurs when the package is cooled below ambient for storage.
Another significant problem that occurs when a product experiences a thermal event, such as either hot-filling a product or heating after filling, as in a pasteurization process, is that the plastic may shrink or distort due to its exposure to the hot product or by the externally added heat during the pasteurization. Pasteurization or sterilization of the package once sealed is customary for comestibles and usually involves spraying the container with hot water, usually at a temperature in the range of 140.degree. F. to about 190.degree. F. The container then goes through a cooling spray process to reduce the bottle temperature to approximately 95.degree. F.
The thermal effects are particularly problematic in one type of container that has received considerable acceptance; a blow-molded bottle that has a hemispherical bottom and is supported through a separate base cup. Containers, according to this design, have found utility, for example, as bottles for soft drinks and other products which traditionally are placed in the container at low or moderate temperatures. However, due to the inherently high orientation of the polymer in a blow molded container, the thermal distortion or shrinkage is high upon exposure to elevated temperature. This thermal distortion can cause many problems with the package integrity.
For example, as a result of the plastic container shrinkage. The head-space volume may be reduced to a point where the product invades the upper portions of the container neck; too close to the mouth of the container. This can cause spillage when the closure is removed by a consumer or may make the product difficult to pour without spillage. This can also cause the fill-level of the product to be hidden by the closure which is generally opaque. This prevents visual inspection of the fill-level height or visual inspection of floating contaminants for quality control purposes.
Thermal distortion can also cause the container to be misshapen which causes problems including: neck finish distortion which can create problems in sealing the containers; bottom surface distortion which can cause the container to lean; and, aesthetic detraction.
Because of the heat distortion and the partial vacuum created in hot-fill applications, it has been customary to employ glass or metal containers for comestible products such as beverages, pharmaceuticals and the like, which are traditionally placed in these containers at an elevated temperature. However, the increased weight and cost of glass bottles or metal containers makes this solution economically unfeasible. Furthermore, glass bottles have an inherent problem of breaking upon sufficient impact.
There have been numerous attempts to solve these problems in stretched blown polyester containers for packaging warm or hot-filled products. For example, the industry has adopted methods of "heat treating" the container, or has attempted to utilize heat-resistant material co-extruded in conjunction with less costly resins. Deflectable container sections have also been employed to manage the negative pressure associated with products hot-filled, then cooled for consumer use.
Heat-treating or heat-setting is taught in a number of patents. Examples are U.S. Pat. Nos. 4,233,022 (Brady); 4,711,624 (Watson); and, 4,219,526 (Mehnert). Similarly, negative pressure management in plastic containers deflectable by use of geometry is taught in a number of recent patents. Examples are U.S. Pat. Nos. 4,318,882 (Argawal); 4,717,525 (Iizuka); and 4,665,682 (Kerins et al.).
In the heat-treating art, the thermoplastic material is formed into a shape and then subjected to heat, whereby the crystalline structure is changed. Generally, the crystalline percentage is increased, thereby increasing the heat distortion resistance of the final shape.
U.S. Pat. No. 4,550,043 (Beck) teaches the use of heat-resistant resin coextruded with other thermoplastics to form a thermally stable molded container suitable for hot-filled products that are pasteurized, while U.S. Pat. No. 4,731,513 (Collette) utilizes a hybrid reheating system to improve the heat distortion resistance of a hot-filled stretch blown container.
Initial attempts were also made for vacuum management in a plastic bottle through a specially-designed bottom which bows inwardly to accommodate the negative pressure. This purely pressure-induced movement, however, is not practical per se to prevent vacuum collapse in polyester containers because it is difficult to manufacture a base which behaves in the proper manner.
In a further effort to control vacuum in a PET container, large collapsible panels in the side wall have been used to
control the size of the container as the internal pressure drops to a negative value. These containers have been proposed as a substitute for glass and have the advantages of lightweight, durable PET containers for a vast new segment of the consumer product packaging industry. Thus, for example, U.S. Pat. Nos. 4,318,882 and 4,717,525 provide a PET container which has at least one region which is thermo-elastically deformable inwardly after the container is hot-filled and sealed to control the vacuum. In both of these patent disclosures, the collapsible panel portions and the remainder of the container are heat-set at different temperatures which increases the time required for forming the container and also therefore increases the cost.
Another patent which shows the method for hot-filling collapsible-resistant polyester containers is disclosed in U.S. Pat. No. 4,665,682. This patent discloses a wide-mouth container that is formed from PET and has inwardly-directed ribs in the side walls to accommodate the vacuum that is produced when the hot-fill product is cooled.
Also, as noted, one prior art solution to the problems associated with thermal distortion of the plastic package involves applying a post-forming heat treatment or "heat set" to the molded container. While it may be possible to reduce the amount of thermal distortion of a PET container by this heat-treatment technique, the resultant container is not suitable for commercial hot-fill temperature applications.
While the methods discussed above appear to provide solutions for the container collapsing problem and thermal distortion problems when hot-filling products in plastic packages such as a PET container, these approaches have not been widely accepted in the management of thermal events like the hot-filling of a product. As is known, the differential heat setting or heat-treatment techniques require considerably longer mold residence time to produce the desired level of thermal conditioning and thereby reduces the productivity of the molding cycle, which is normally done in a two-stage, high-output intermittent, continuous motion, reheat-blow-molding machines.
Furthermore, the collapsible containers disclosed in the above patents are generally aesthetically unacceptable because it is difficult to apply the desired labeling on the finished container. With the larger collapsible control panels molded in the side walls, it is not practical to utilize the side wall of the container for the label.
If the side wall is used to support the label, the label cannot be adhered because the collapsing of the side wall will distort the label. If the label is not adhered, it can easily be torn or disfigured. For many hot-filled products, it is desirable to have the major portion of the container covered by the label and the non-circular nature of the prior art containers prohibits such type of labeling.
It should be noted that these problems are not found generally in soft drink packaging. Most soft drinks are carbonated and filled at lower-to-moderate temperatures so heat distortion is generally not problematic. Furthermore, it is noted that the internal pressure caused by the carbonation helps to prevent the flexible polyester container from collapsing during shipping and handling.
Thus, it has been recognized that in the packaging of products that are warm or hot-filled (80.degree. F. to 200.degree. F.), the thermoplastic package must have the ability to resist thermal distortion and the package must contain the negative pressure formed upon cooling. However, the hot-filling industry has not recognized that it is possible to achieve these results by providing a small quantity of a liquified gas to the contents of the hot-filled plastic container in a manner that capitalizes on the gas expansion to (1) offset the thermal distortion; and, (2) neutralize the negative pressure and form a positive pressure at storage and use conditions.
The use of positive pressure to rigidize a sealed deformable metal container is a well-known art. For instance, U.S. Pat. No. 3,699,740 (Knobe) has shown that it is possible to prevent container deformation during an air evacuation process in the filler that removes air from sensitive products to protect the flavor, while circulating an inert gas through the container at a pressure above the container deformation pressure during the filling operation.
Similarly, metal containers that are hot-filled are prevented from deformation, caused by a negative pressure that is developed upon cooling, through the addition of liquid nitrogen prior to sealing. Examples are Canadian Patent No. 1,062,671 (Cook) and U.S. Pat Nos. 2,978,336 (Morrison) and 4,583,346 (Kameda), issued to the Assignee. In the Morrison teaching, edible material is flushed with liquified nitrogen injected adjacent the bottom of the package and allowed to expand prior to sealing, thereby expelling the air. Cook provides a method that injects a small drop of liquid nitrogen into a filled metal can that is subsequently sealed. The expansion of the liquid nitrogen creates an internal pressure in the head space which increases the mechanical strength of the metal container.
However, none of the above references completely recognize the problems associated with hot-filling or thermal processing products in plastic containers and do not teach or suggest the use of introducing liquified gas into the plastic containers and sealing the container before any significant distortion occurs.